The present invention relates to direct current motors having permanent magnets.
A conventional direct current (d.c.) motor 20 is comprised of permanent magnets 21 and 22, an armature 23, a commutator 24, brushes 25 and the like as shown in FIGS. 13A to 13C. In this motor, the armature 23 rotates as shown in the order of FIGS. 13A, 13B and 13C, when direct current power is supplied thereto.
Specifically, the armature 23 has an armature core 26 and armature coils 27. A plurality of teeth 26a is formed on the core 26. Each coil 27 is wound around five teeth 26a, although only one is shown in the figures. The coils 27 are wound in a distributed winding form.
The commutator 24 has a plurality of segments 24a on which the bushes 25 slide, so that the direct current flows from the brushes 25 to the coils 27 through the segments 24a of the commutator 24. Thus, the armature 23 rotates in the clockwise direction (arrow X) in the figures, as the direction of current flowing in the coils 27 is reversed.
The current supplied to the coil 27 from the brush 25 is changed as shown in FIGS. 14A to 14C. It is assumed that the current I flows from right to left as shown in FIG. 14A, and that the commutator 24 moves to the right as shown in FIG. 14B relative to the brush 15 as the armature 23 rotates. The brush 25 bridges two adjacent segments 24a to supply the coil 27 with shorting current i. The current I flows from the left to the right in the coil 27 as shown in FIG. 14C, as the armature 23 rotates further. That is, the direction of the current I flowing in the coil 27 is reversed, when the armature 23 rotates as shown in the order of FIGS. 14A, 14B and 14C. In this instance, the current which changes by 2I from +I to −I is supplied from the brush 25.
FIGS. 14A to 14C corresponds to FIGS. 13A to 13C. When the armature 23 rotates as shown in the order of FIGS. 13A, 13B and 13C,the direction of current I in the soil 27 is reversed. The direction of the magnetic field in the core 26 wound with the core coil 27 is reversed. The rotating force is generated to rotate the armature 23 by the electromagnetic force of the coils 27 and the magnetic force of the magnets 21 and 22.
The reversion of current flowing in the coil 27 during the period of shorting by the brush 25 is defined as commutation. This relation is expressed in the following commutation equation.L(di/dt)+e+Rci+R2(I+i)−R1(I−i)=0
In the above equation, L(di/dt) is a reactance voltage generated by an inductance of the coil 27 shorted by the brush 25, and e is an induction voltage generated in the coil 27 when the armature 23 rotates. Rc is a resistance of the coil 27 shorted by the brush 25. R1 and R2 are contact resistances between the brush 25 and the commutator 24. I is a current supplied form the brush 25, and i is a shorting current of the coil 27 shorted by the brush 25.
The shorting current i changes linearly as shown by the dotted line in FIG. 15, as long as the reactance voltage L(di/dt) of the coil 27 and the induction voltage e is negligible during the commutation period. In this instance, the commutation is effected linearly and most favorable.
However, the reactance voltage and the induction voltage are generated in the coil 27 in fact. The shorting current i therefore flows with a delay in time relative to the linear commutation characteristics as shown by the solid line in FIG. 15, resulting in an insufficient commutation. This insufficient commutation causes spark discharges at the rear end of the brush 25, when the commutation terminates. The spark discharges causes noise and brush wear.
It is proposed to counter this problem, that is, improve the commutation operation by moving the brush in the counter-clockwise direction in FIG. 13. The brush is moved to reduce the influence of the induction voltage e. Specifically, the induction voltage e is generated as a counter-electromotive force in the coil 27 by changes in the magnetic flux amount Φ passing through the coil 27. This voltage e is expressed as follows.e=−dΦ/dt
That is, the induction voltage e is generated in proportion to the speed of reduction in the magnetic flux amount Φ passing through the coil 27.
The induction voltage e is shown in FIG. 16. Specifically, FIG. 16 shows changes in the magnetic flux amount Φ passing through the coil 27 and hence passing through the core 26 (five teeth 26a) around which the coil 27 is wound, and the induction voltage e generated in the coil 27 in response to the change in the magnetic flux amount Φ. In FIG. 16, the magnetic flux amount Φ and the induction voltage e are shown with respect to a reference position (0°) which corresponds to FIG. 13B. That is, the reference position is defined as the position where the center of the core 26 (five teeth 26a) wound with the coil 27 coincides with the center of the magnet 21 or 22.
When no current flows in the coil 27, only the magnetic flux of the permanent magnets 21 and 22 passes through the coil 27. In this instance, the magnetic flux amount Φ is maximal when the rotation position of the armature 23 is at the reference position (FIG. 13B) as shown by (A) in FIG. 16.
When the current flows in the coil 27, however, it generates the magnetic force which influence the magnetic flux of the magnets 21 and 22. As a result, the magnetic flux amount Φ of the coil 27 changes with the rotation position of the armature 23 as shown by (B) in FIG. 16, because the current is reversed during the commutation period, that is, when the armature 23 rotates as shown in the order of FIGS. 13A, 13B and 13C. That is, the magnetic flux amount Φ changes from positive to negative, when the armature 23 passes through the reference position. As a result, the magnetic flux amount Φ which actually passes through the coil 27 changes as shown by (C) in FIG. 16. This amount is a sum of the flux amounts indicated by (A) and (B). Thus, the actual magnetic flux amount Φ becomes maximum before the armature 23 rotates to the reference position. As a result, the induction voltage e of the coil 27 changes from negative to positive when the total magnetic flux amount becomes the maximum. For this reason, the induction voltage e is generated in a manner to delay the commutation and delay the reversion of the shorting current i, causing the insufficient commutation.
Therefore, the influence of the induction voltage e in the coil 27 is minimized by moving the position of the brush 25 in the direction opposite the rotation of the armature 23, that is, in the counter-clockwise direction in FIG. 13. In practice, the position of the brush 25 is determined based on not only the induction voltage but also the reactance voltage.
It is however difficult to maintain good commutation operation, because the current flowing in the coil 27 and the rotation speed of the motor change from time to time. For instance, in the case of a blower motor used for an automotive air conditioner unit, the position where the total magnetic flux amount Φ attains the maximum moves to a position (negative side in FIG. 16) opposite the rotation direction at high load and high rotation speed conditions because more current is supplied. The induction voltage e caused by the total magnetic flux amount Φ also increases as the rotation speed increases. Further, the reactance voltage also increases as the current in the coil 27 increases. Thus, the brush need be moved more for good commutation operation. In the case of low load and low speed conditions, on the contrary, the brush need be moved less for good commutation operation. It is thus required to move the brush position from time to time.